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Types of Tunnels & Construction Methods
(a) Mountain Tunnel
-- Drilling and blasting (D&B) method: NATM (New Austrian Tunneling Method) is the most
common method. It originates in hard rock tunneling and utilizes rockbolts and shotcrete applied
immediately after blasting. This is often followed by a cast in-situ concrete lining using formwork.
-- Tunnel Boring Machine (TBM) method: TMB is used as an alternative to drilling and blasting
(D&B) methods. TBMs are used to excavate tunnels with a circular cross section through a variety
of subterranean matter; hard rock, sand or almost anything in between. As the TBM moves
forward, the round cutter heads cut into the tunnel face and splits off large chunks of rock. The
cutter head carves a smooth round hole through the rock -- the exact shape of a tunnel. Conveyor
belts carry the rock shavings through the TBM and out the back of the machine to a dumpster.
Tunnel lining is the wall of the tunnel. It consists of precast concrete segments that form rings,
cast in-situ concrete lining using formwork or shotcrete lining.
(b) Shallow-buried Tunnel or Soft Soil Tunnel
Shallow tunnels are of a cut-and-cover type (if under water of the immersed-tube type). Deep
tunnels are excavated, often using a tunnelling shield. For intermediate levels, both methods are
possible.
-- Cut-and-cover method: Cut-and-cover is a method of tunnel construction where a trench is
excavated and roofed over. Strong supporting beams are necessary to avoid the danger of the tunnel
collapsing.
-- Shield method: The Shield method uses one or two shields (large metal cylinder) to cut out a
tunnel through the soft ground. A rotating cutting wheel is located at the front end of the shield.
Behind the cutting wheel is a chamber where, depending on the type of the TBM, the excavated
soil is either mixed with slurry (called slurry TBM) or left as is (earth pressure balance or
EPB shield). Systems for removal of the soil (or the soil mixed with slurry) are also
present.
Behind the chamber is a set of hydraulic jacks supported by the finished part of the
tunnel which are used to push the TBM forward. Once a certain distance has been
excavated (roughly 1.5-2 meters), a new tunnel ring is built using the erector. The
erector is a rotating system that picks up pre-cast concrete segments and places
them in the desired position. Behind the shield, inside the finished part of the tunnel, several
support mechanisms can be found that are part of the TBM: dirt removal, slurry pipelines if
applicable, control rooms, and rails for transport of the precast segments, etc.
(c)Underwater Tunnel
-- Immersed-tube method: The immersed tube tunnel technique uses hollow box sectioned tunnel
elements that have been prefabricated in reinforced concrete. These are floated out into the harbor
and placed into a trench that was pre-dredged in the harbor bed. When in position,
the elements are joined together to form a tunnel. The trench is then refilled and the harbour bed
returned to its original level.
-- Shield method: As previously stated.
Tunnel Construction Methods and their comparison
1. Introduction – This paper gives a general description of the tunnelling techniques such as cut
and cover, drill and blast, bored tunnelling and sequential mining construction, reviewed for
possible use in various projects.
A summary of environmental merits and demerits associated with these methods are also given.
Above tunnelling techniques are mostly used to construct small tunnels and find their applications
in utility projects to a great extent.
2. Construction Methods:
a. Cut and Cover Tunnelling – Cut and cover tunnelling is a common and well-proven technique
for constructing shallow tunnels. The method can accommodate changes in tunnel width and non-
uniform shapes and is often adopted in construction of underground stations. Several overlapping
works are required to be carried out in using this tunnelling method. Trench excavation, tunnel
construction and soil covering of excavated tunnels are three major integral parts of the tunnelling
method.
Most of these works are similar to other road construction except that the excavation levels
involved are deeper. Bulk excavation is often undertaken under a road deck to minimise traffic
disruption as well as environmental impacts in terms of dust and noise emissions and visual impact.
b. Drill and Blast – This tunnelling method involves the use of explosives. Drilling rigs are used
to drill blast holes on the proposed tunnel surface to a designated depth for blasting. Explosives
and timed detonators (Delay detonators) are then placed in the blast holes. Once blasting is carried
out, waste rocks and soils are transported out of the tunnel before further blasting. Most tunnelling
construction in rock involves ground that is somewhere between two extreme conditions of hard
rock and soft ground. Hence adequate structural support measures are required when adopting this
method for tunnelling.
Compared with bored tunnelling by Tunnel Boring Machine (see below), blasting generally results
in higher but lesser duration of vibration levels. A temporary magazine site is often needed for
overnight storage of explosives.
c. Bored Tunnelling by Tunnel Boring Machines (TBM) – Bored tunnelling by using a Tunnel
Boring Machine (TBM) is often used for excavating long tunnels. An effective TMB method
requires the selection of appropriate equipment for different rock mass and geological conditions.
The TBM may be suitable for excavating tunnels which contain competent rocks that can provide
adequate geological stability for boring a long section tunnel without structural support. However,
extremely hard rock can cause significant wear of the TBM rock cutter and may slow down the
progress of the tunnelling works to the point where TBM becomes inefficient and uneconomical
and may take longer time than the drill-and-blast tunnelling method.
d. Sequential Excavation Method – This method is also known as the New Austrian Tunnelling
Method (NATM). The excavation location of a proposed tunnel is divided into segments first. The
segments are then mined sequentially with supports. Some mining equipments such as road-
headers and backhoes are commonly used for the tunnel excavation. The ground for excavation
must be fully dry for applying the NATM and ground dewatering is also an essential process before
the excavation. Another process relates to the ground modifications such as grouting and ground
freezing is also common with this method in order to stabilize the soil for tunnelling. This method
is relatively slow but is found useful in areas where existing structures such as sewer or subway
could not be relocated.
3. Environmental Merits and Demerits – Selection of the techniques to be adopted for
construction of a tunnel section shall take into account the nature of the substrata and the levels of
the tunnel involved. A summary of the environmental advantages and disadvantages associated
with the construction methods is tabulated below:
Tunnel Construction Environmental advantages and disadvantages
Methods (on relative terms)
Disadvantages:
More dust and noise impact may arise, though
these can be mitigated through implementation
of sufficient control measures;
Temporary decks are often installed before bulk
excavation to minimise the associated
environment impacts;
Larger quantity of C&D materials would be
generated from the excavation works, requiring
Cut and cover tunnelling proper handling and disposal.
Advantages:
Potential environmental impacts in terms of
noise, dust and visual on sensitive receives are
significantly reduced and are restricted to those
located near the tunnel portal;
Compared with the cut-and-cover approach,
quantity of C&D materials generated would be
much reduced;
Compared with the cut-and-cover approach,
disturbance to local traffic and associated
environmental impacts would be much
reduced;
Blasting would significantly reduce the
duration of vibration, though the vibration level
would be higher compared with bored
tunnelling (with proper blast design &
techniques vibration can be reduced);
Drill and blast
Disadvantages:
Potential hazard associated with establishment
of a temporary magazine site for overnight
storage of explosives shall be addressed
through avoiding populated areas in the site
selection process.
Advantages:
Potential environmental impacts in terms of
noise, dust and visual on sensitive receives are
significantly reduced and are restricted to those
located near the launching and retrieval shafts;
Compared with the cut-and-cover approach,
disturbance to local traffic and associated
environmental impacts would be much
reduced;
Compared with the cut-and-cover approach,
quantity of C&D materials generated would be
Bored tunnelling by TBM much reduced;
Advantages:
Similar to the drill-and-blast and bored
tunnelling methods, only localised potential
environmental impacts would be generated;
Disadvantages:
As the method is relatively slow, duration of
Sequential Excavation potential environmental impacts would be
Method longer than that of the other methods.
Tunnel Excavation by Tunnel Boring Machine (TBM) & Parameters
Major components of this Tunnel Boring Machine include:
1. Cutter head, with cutting discs/tools and
2. Muck buckets to carry and dispose excavated muck
3. Power supply Systems
4. Cutter head rotation & thrust
5. Bracing system for the TBM during mining
6. Equipment for ground support installation
7. Shielding to protect workers
8. Steeling system
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